1. Field of the Invention
This invention relates to helicopter rotors having an even number of blades wherein the opposed blades have a common member in the form of a flexible spar extending across the rotor axis. Such a rotor is identified herein as a cross-beam rotor. The spar is preferably fabricated of uni-directional, high tensile strength fibers bonded together and substantially rectangular in cross section. Improvements to the configuration of the spar cross section may result in a change to its shape, but are considered apart from the teaching of this invention, which can accommodate various shapes. The spar reacts the centrifugal load between opposite blades and may extend the full span of both blades to each tip, or may terminate at a blade attachment connection at a convenient radial station. The full tip-to-tip spar embodiment is generally limited to helicopter tail rotor applications, since a main rotor spar length equal to the rotor diameter would be too cumbersome to handle in the field as a single unit. The spar section is designed to provide the desired degree of stiffness in the in-plane and flapping directions for blade bending as well as torsional flexibility for blade pitch change motions. Pitch motions are introduced into the blade by twisting of the spar member. This invention recognizes the benefits of the prior art configurations that allow twisting and bending of the spar between bearing supports located on opposed sides of the rotor axis, and further improves such benefit by providing means for the spar to bend and twist over a further distance; that is, beyond the bearing supports to the radial attachment station where the spar-blade attachment joint is located. Thus, when spar bending and twisting occurs to its maximum degree, our improvement will result in smaller angular deflection of the spar and attendantly smaller stress levels. When used in a main rotor application, the blade radial station attachment joint is chosen so that the spar will have a long enough free length to accommodate all deflections at low stress and at the same time be short enough to allow a maximum length of the blade span to form the airfoil contour.
2. Description of the Prior Art
In its earliest form, the development efforts of the assignee of this application towards elimination of conventional rotor stack bearings in favor of a flexible cross-beam rotor took the form of the embodiment depicted in its Canadian Patent to Arcidiacono No. 951, 301 filed Nov. 10, 1971 and issued July 16, 1974. In Arcidiacono, the flexible spar straps were sandwiched between two hub plates, and pitch inputs were introduced by means of a shell which attached to the flexible strap at a point approximately 30 percent of the rotor radius. Blade pitch control inputs twisted the strap inboard to the radial station where it was rigidly clamped by the hub members. The slotted hub plates into which the blade spar members are recessed provides means to impart drive between the drive shaft connected hub plates and the blades. The U.S. Pat. No. 3,484,174 to McCoubrey illustrates a second embodiment of a cross-beam rotor with a gimbaled flapping hinge. The connection of the flexible member to the hinge member provides for similar limited torsional deflection as in Arcidiacono, in that the spar portions are also rigidly clamped. Drive means is imparted from the drive shaft through a splined connection in the yoke assembly to an irregularly shaped insert and collar adapted to bear against the center portion of the spar in its spread apart relationship as it passes around the rotor axis.
The assignee's U.S. Pat. No. 3,874,820 to Fenaughty illustrates a further development in the field of this invention and is addressed principally to the angular relationship between the airfoil and supporting spar member. While the blade supporting structure is not shown, the structure utilized for flight aircraft comprised upper and lower clamp members which functioned similar to the structure shown in the Arcidiacono Patent. A still further development is presented in assignee's U.S. Pat. No. 4,008,980 to Hibyan and Noehren which shows the support structure referred to in the above reference to the Fenaughty patent. This structure is further described in the paper entitled: "Composite Bearingless Tail Rotor for UTTAS" dated May 1976 by R. Fenaughty and W. Noehren presented at the 32nd Annual National V/STOL Forum of the American Helicopter Society. The U.S. Pat. No. 3,874,815 to Baskin advanced the field of art by introducing flexible transverse pads in place of either the rigid clamps of Arcidiacono and others, or the metal gimbal bearings of McCoubrey. By means of the bearing pad or flexure member located transverse to the spar member, flexibility is provided to allow bending deflections caused by blade flapping to take place inboard of the radial position of the bearing pad; a feature not provided for theretofore. Similar to Arcidiacono, the spar members of Baskin are captured on their edges between bolted hub plates, the lower of which is connected to the drive shaft.
More recent U.S. Pat. No. 4,053,258 to Mouille further illustrates the known rigid clamping connection between the cross-over spar member and opposed hub plates.
The most significant and recent development in this field known to the applicants is the work of Rybicki as described in his U.S. Pat. No. 4,093,400, wherein his cross-beam spar is supported from the rotor hub arms by spherical elastomeric bearings, thus eliminating all need to twist the spar member when a cyclic path input is introduced. The advancing blade angle of attack is increased and the retreating blade angle of attack is reduced simply by tilting of the spar. This advantage is accomplished without sacrifice of the benefit of the advantage illustrated in Baskin; namely, the allowance of flap-wise spar bending across the rotor axis.